As this recent story from the Sacramento Bee reports, the need for major new water storage capacity on the American River to protect Sacramento from flooding as well as protecting the integrity of the American River Parkway, is apparent.
“The past two weeks offered a powerful reminder, as both the Sacramento and American rivers have swelled substantially in response to a stormy December. Such high flows haven't been seen since the winter of 2005-06.
“For some, it's an inconvenience. Parts of the American River Parkway bike path went underwater two weeks ago when releases from Folsom Dam were doubled – to 30,000 cubic feet per second – to make way for upstream runoff. This forced some bicycle commuters to find a detour….
“December has been very wet in California. So wet that, as of Wednesday, the statewide snowpack stood at 215 percent of average for the date. But it has not been wet enough yet to approach flooding conditions on any of the state's major rivers.
“Those big releases from Folsom Dam two weeks ago? That was nothing compared to a true flood-control scenario.
“The American River channel has a current flood-flow capacity nearly four times greater, or about 115,000 cfs. That volume hasn't been approached since 1997, the last year of serious flood risk.
“But even 30,000 cfs gets attention. And it's still a lot of water: enough to fill an Olympic-size swimming pool every three seconds.
“At those flows, the river is transformed. Shorelines where we normally walk, fish or picnic are gone, replaced by fast, cold water. Trees go under water. Wading beaches become distant and dangerous rapids….
“Extra water had to be released from Folsom Dam this month because, during winter, federal rules require a certain capacity to be maintained in reservoirs to capture potential floods. The numbers vary, but Folsom Dam's current storage target is 397,500 acre-feet, or just 40 percent of its total capacity. The current actual storage is 431,171 acre-feet.
“Folsom is managed more actively for flood protection than most other major reservoirs in California for several reasons.
“One is that it is relatively small compared to the size of the upstream watershed. This means a storm making a direct hit on the watershed could quickly overwhelm the reservoir.
“Another is that it flows into an even bigger river – the Sacramento – at a confined location in the heart of a major metropolitan area.
“When the Sacramento River is full – which often occurs simultaneously with the American – the additional flow from Folsom Dam can simply back up between the levees, creating additional flood risk.
“So early and active management of Folsom Dam flows is critical to public safety.
"It's one of the more likely reservoirs to reach flood control (status) during any winter," said Maury Roos, chief hydrologist at the California Department of Water Resources.
“In comparison, Shasta and Oroville reservoirs have not made any significant flood control releases yet. That is a measure both of their much larger size and also of the merely middling severity of the winter so far.”